What are the seven SI base units?

Metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). Every other SI unit is derived as a product of powers of these seven.

What changed in the 2019 SI redefinition?

On 20 May 2019 the base units were redefined so each is fixed by an exact value of a fundamental constant rather than a physical artefact. Most notably the kilogram is now defined via the Planck constant instead of a platinum-iridium cylinder kept in France.

Why was the old kilogram prototype a problem?

The international prototype kilogram was a physical object whose mass could drift over time through surface contamination or cleaning. Tying the kilogram to the Planck constant makes it reproducible in any properly equipped laboratory and stable forever.

How is the second defined?

The second is defined by setting the caesium-133 hyperfine transition frequency to exactly 9 192 631 770 hertz. One second is the time for that many oscillations, which is the basis of atomic clocks.

Why is the candela in the base set?

Luminous intensity is weighted by the human eye's response to light, so it cannot be derived purely from energy units. The candela captures perceived brightness and is defined by fixing the luminous efficacy of green light at 540 terahertz.

SI Base Units Reference

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The International System of Units is built on seven base units, each measuring a distinct physical quantity. Every other unit in the SI — from the newton to the volt — is a product of powers of these seven. This reference gives each unit’s symbol and its modern, constant-based definition.

How it works

Pick a base quantity and the tool shows its unit, symbol, and the defining relationship adopted in the 2019 SI redefinition. Before 2019 the kilogram was defined by a physical metal cylinder; since then every base unit has been fixed by an exact numerical value of a constant of nature:

metre via the speed of light c
kilogram via the Planck constant h
second via the caesium-133 hyperfine frequency
ampere via the elementary charge e
kelvin via the Boltzmann constant k
mole via the Avogadro constant NA
candela via the luminous efficacy of 540 THz light

Anchoring units to constants makes them reproducible in any lab and immune to the drift that affected the old physical prototypes.

Notes and examples

The redefinition was invisible in everyday life — a kilogram of sugar still weighs the same — but it future-proofed metrology. A standards lab anywhere can now realise the kilogram from first principles using a Kibble balance, rather than comparing against a single artefact in Paris. The seven base units and their symbols (m, kg, s, A, K, mol, cd) are worth memorising because all derived units trace back to them.